1. Field of the Invention
This invention relates to an extrusion process for conversion of thermoplastic polymer billets into extruded shapes with smooth, unbroken surfaces.
This invention also relates to a solid-state deformation process, and more particularly to a process for solid-state extrusion of thermoplastic polymer billets into oriented large extruded shapes at high extrusion rates.
Most polymers have a chain of carbon to carbon bonds along their backbone. Upon solidification of some polymers, a portion of the polymer chains in the material are folded to form crystals which are randomly oriented within the material. If even a small portion of the material behaves in this manner, the material is referred to as semi-crystalline. Such material may deform easily by bending, sliding and breaking of the crystals or a small fraction of the non-crystalline entangled molecular chains. If the chains are aligned or oriented, the mechanical strength is much improved. It is well known that properties such as strength and stiffness are enhanced by aligning or oriented the polymer chains. One technique for orienting some polymers, such as polyethylene, is by plastic flow at temperatures below the melting point. The degree of orientation in such polymers is indicated by the increase in tensile modulus of the oriented polymer over the tensile modulus of the unoriented polymer.
Processes for the solid state deformation of polymers are well known. Among the processing techniques used to make profiles of polymers are ram and hydrostatic extrusion. In ram extrusion the billet of polymer is placed inside a usually cylindrical pressure chamber, so that the surface of the billet is in immediate contact with the walls of the chamber. One end of the chamber is fitted with a die, whose opening corresponds to the profile it is desired to produce. The other end of the pressure chamber is closed by an axially mobile ram, attached to a hydraulic system, so arranged that the ram pushes against the billet of the polymer and forces the polymer out from the chamber by flowing through the die.
In hydrostatic extrusion on the other hand, the billet is much smaller than the pressure chamber, and the surface is separated by some distance from the chamber wall. The intervening space is filled with a hydraulic fluid. One end of the chamber is fitted with a pressure generating device, which may be a piston, or by an inlet through which hydraulic fluid is pumped into the chamber. The other end of the chamber is fitted with the die. One end of the billet is machined in such a way that the nose piece fits into the throat of the die, and makes a liquid tight seal. During extrusion, the pressure on the hydraulic fluid is increased. This pressure is transmitted in both the axial direction and the radial direction to the billet, so that it is pressurized equally in all directions. As a consequence, the surface of the billet is in contact with the oil, and some of the oil adheres to the surface of the billet as it passes through the die, providing a significant amount of lubrication.
During solid state deformation processing such as rolling, drawing and extrusion, the polymers typically lose the spherulitic aspect generated during the cooling form the molten state, and become oriented usually in a longitudinal direction. The orientation of the polymer in a longitudinal direction increases the mechanical properties of the polymer, e.g. its tensile strength and stiffness. These are sought after properties. One disadvantage of oriented polymers is that they are weak in the transverse direction, and are subject to transverse cracking under stress, or to fibrillation under abrasion.
At the same time, the process of pushing the polymer through the chamber and the die creates surface imperfections which range from superficial defects, to micro cracks that extend a small distance into the profile, to much larger cracks and fissures. These are undesirable surface imperfections. They are caused by the frictional forces existing between the advancing polymer being pushed by the ram, and the surface of the metal of the pressure chamber, the die and the die land.
It is desirable to reduce these frictional forces, and at the same time prevent or remedy the defects that are produced during movement of the polymer through the extrusion apparatus. Furthermore, it is desirable to increase the toughness of the extruded polymer. Toughness is related to the degree of orientation of the polymer, the lower the degree or orientation, the higher the toughness in the transverse direction.
2. Description of the Related Art
Various prior workers have extruded semi-crystalline polymers by the ram extrusion method. For instance, Matsuo et al (U.S. Pat. No. 4,731,199) disclose that the use of lubricants of the acid ester type reduces the pressure required to extrude pipe profiles of sintered ultra high molecular weight polyethylene with a resulting improvement in the dimensional uniformity of the extrusion.
Ramamurthy (U.S. Pat. No. 4,554,120) discloses that use an alloy of Cu/Zn alloy in the land region of the die to replace the mild steel usually employed increases the adhesion of the polyethylene polymer to the die, and thereby improves the surface of thermoplastic extrusions. Ramamurthy (U.S. Pat. No. 4,552,712) discloses that use of stainless steel in the land region of the die to replace the mild steel usually employed, and the use of an additive to the polymers increases the adhesion of the polyethylene polymer to the die, and thereby improves the surface of thermoplastic extrusions.
Hureau et al (U.S. Pat. No. 4,303,609) disclose that an extrudate of a thermoplastic polymer can be modified by heating it after exit from the die, by blowing hot air from nozzles onto its surface such that the polymer melts.
Nippon Steel Chem KK (Japanese Patent Application 63 161075) discloses that the surface of the extrudates of polyethylene can be improved by adding a lubricant to the polyethylene polymer before extrusion.
Japanese Patent Application 52 148,562 discloses heating the surface of an extruded polyethylene with a gas burner, in order to make the surface accept a conducting material.
Chung et al "Energy Efficiency in Plasticating Screw Extrusion", American Chemical Society, Preprints v. 39, for National Meeting of American Chemical Society, 176th, Miami Beach, Sep. 10-15, 1978, Published by the American Chemical Society, Division of Organic Coatings and Plastic Chemicals, Washington, D.C., 1978, p 74 to 80, discloses that when a solid plug of polymer is rubbed on a heated barrel surface, a thin layer of melt or melt film develops between the solid plug and the barrel surface. The Chung paper does not propose the use of this heating and melting to improve the properties of the extrudate, rather their interest is to use the molten polymer as a lubricant, and thereby reduce the energy required to process and melt the polymer.